A four-wave mixing experiment in a bulk InGaAsP traveling-wave semicon
ductor amplifier is reported. The maximum pump-probe detuning is 4.3 T
Hz. The equivalent time resolution of 37 fs is high enough to measure
with good accuracy the time constant of spectral-hole burning (100 fs
in our case). The simultaneous presence of spectral-hole burning and o
f an instantaneous, within our time resolution, saturation process is
clearly displayed.